1. ECE 8443 – Pattern Recognition ECE 3163 – Signals and Systems Objectives: Stability and the s-Plane Stability of an RC Circuit Routh-Hurwitz Stability Test Step Response of 1 st -Order Systems Resources: MH: Control Theory and Stability Wiki: Routh-Hurwitz Stability Test TBCO: Routh-Hurwitz Tutorial EC: Step Response MH: Control Theory and Stability Wiki: Routh-Hurwitz Stability Test TBCO: Routh-Hurwitz Tutorial EC: Step Response LECTURE 31: STABILITY OF CT SYSTEMS Audio: URL:

2. ECE 3163: Lecture 31, Slide 1 Stability of CT Systems in the s-Plane Recall our stability condition for the Laplace transform of the impulse response of a CT linear time-invariant system: This implies the poles are in the left-half plane. This also implies: A system is said to be marginally stable if its impulse response is bounded: In this case, at least one pole of the system lies on the j  -axis. Recall periodic signals also have poles on the j  -axis because they are marginally stable. Also recall that the left-half plane maps to the inside of the unit circle in the z- plane for discrete-time (sampled) signals. We can show that circuits built from passive components (RLC) are always stable if there is some resistance in the circuit. LHP

3. ECE 3163: Lecture 31, Slide 2 Stability of CT Systems in the s-Plane Example: Series RLC Circuit Using the quadratic formula: The RLC circuit is always stable. Why?

4. ECE 3163: Lecture 31, Slide 3 The Routh-Hurwitz Stability Test The procedures for determining stability do not require finding the roots of the denominator polynomial, which can be a daunting task for a high-order system (e.g., 32 poles). The Routh-Hurwitz stability test is a method of determining stability using simple algebraic operations on the polynomial coefficients. It is best demonstrated through an example. Consider: Construct the Routh array: Number of sign changes in 1 st column = number of poles in the RHPRLC circuit is always stable

5. ECE 3163: Lecture 31, Slide 4 Routh-Hurwitz Examples Example:

6. ECE 3163: Lecture 31, Slide 5 Analysis of the Step Response For A 1 st -Order System Recall the transfer function for a 1 st -order differential equation: Define a time constant as the time it takes for the response to reach 1/e (37%) of its value. The time constant in this case is equal to -1/p. Hence, the real part of the pole, which is the distance of the pole from the j  -axis, and is the bandwidth of the pole, is directly related to the time constant. num = 1; den = [1 –p]; t = 0:0.05:10; y = step(num, den, t);

7. ECE 3163: Lecture 31, Slide 6 Summary Reviewed stability of CT systems in terms of the location of the poles in the s-plane. Demonstrated that an RLC circuit is unconditionally stable. Introduced the Routh-Hurwitz technique for determining the stability of a system: does not require finding the roots of a polynomial. Analyzed the properties of the impulse response of a first-order differential equation. Next: analyze the properties of a second-order differential equation.